Among various printing or recording processes, electrophotographic process comprises charging a photoconductive photoreceptor, imagewise exposing the photoconductive photoreceptor to light to form an electrostatic latent image thereon, developing the electrostatic latent image with a particulate toner comprising a coloring agent, etc., transferring the toner image onto a recording medium such as recording paper, and then fixing the toner image thus transferred.
In such an electrostatic image recording process, the surface of the photoreceptor from which the toner image has been transferred is freed of residual particulate toner and destaticized for possible subsequent prolonged repeated use. Accordingly, the foregoing photoreceptor needs to have resistance to inert gas produced during charging such as ozone and NOx (corona resistance), not to mention excellent electrophotographic properties. In particular, the photoreceptor must satisfy requirements for durability and abrasion resistance during repeated use.
In recent years, with the development of data apparatus, a laser beam printer has been developed which uses laser beam to expose the photoconductive photoreceptor to light so that a modified signal as indicated by computer is given to reproduce the recorded image in dot form. In particular, the recent trend is for laser beam printers to form images with even higher quality. Accordingly, the laser beam to be used in the recent laser beam printers is reduced in diameter to raise the resulting dot density to as high as from 600 dpi (dots/inch) to 1,200 dpi. With this trend, there has been a growing demand for enhancement of durability and abrasion resistance of photoreceptors for the purpose of retaining these fine electrostatic latent images.
As electrophotographic photoreceptors there have heretofore been often used inorganic photoconductive photoreceptors made of amorphous silicone, selenium or the like. In recent years, however, the trend is toward the use of organic photoconductive photoreceptors (hereinafter referred to as "organic photoreceptors"), which can be produced at less cost, give no toxicity and are sensitive to the wavelength range of exposing light, particularly to the long wavelength range such as semiconductor laser wavelength range, to be used.
The fatigue deterioration of the foregoing photoreceptor after repeated use is possibly attributed to abrasion and damage on the surface of the photoreceptor developed when it is rubbed at the step of separating and transferring the toner image formed thereon onto the recording medium and removing residual toner from the photoreceptor and denaturation and decomposition of the surface layer of the photoreceptor at the step of charging, exposure and destaticization of the surface of the photoreceptor.
Accordingly, in order to prevent the fatigue deterioration of the foregoing photoreceptor, it is important to improve the surface layer of the photoreceptor. In particular, organic photoreceptors are softer than inorganic photoreceptors and comprise an organic material as a photoconductive material and thus are liable to drastic fatigue deterioration after repeated use. Thus, the improvement of the surface layer is more important for these organic photoreceptors.
On the other hand, processes for developing electrostatic latent image with a toner can be roughly divided into two groups, i.e., binary development process using a binary developer comprising a toner and a carrier and unitary development process using a toner alone. Various proposals have been made for each of the two development processes. In particular, laser beam printers often use a binary developer comprising a toner and a carrier to satisfy the requirements for higher recording speed and higher image quality. The recent trend is for binary developers having a smaller particle diameter to be used. The application of a particulate toner having a volume-average particle diameter of not more than 10 .mu.m and a particulate carrier having a weight-average particle diameter of not more than 100 .mu.m has been under way.
In the electrostatic recording process, the step of fixing a toner image on the recording medium is important. Examples of toner image fixing processes which have heretofore been frequently used include heat roller fixing process having so high a thermal efficiency as to provide a high speed fixing and heat fixing process using a heat source such as oven and flash lamp. For the heat roller fixing process in particular, the development of a toner which can be fixed at a reduced power consumed for fixing heater and hence a lowered heat roller temperature has been desired to satisfy the following requirements:
(1) To inhibit overheat deterioration of printer and hence heat deterioration of parts thereinside;
(2) To shorten the warming-up time between the time at which the fixing device is actuated and the time at which fixing is made possible; and
(3) To inhibit unthorough fixing due to absorption of heat by the recording paper, making it possible to form images on continuous paper.
The use of a finely particulate toner having a particle diameter of not more than 10 .mu.m causes the following troubles. The use of a finely particulate toner in the development process can provide a high resolution and a high dot density reproducibility, making it possible to reduce the amount of the toner required to obtain the same image density. However, such a toner has an increased specific surface area that increases chargeability per unit weight of the toner, possibly causing deterioration of image density.
Further, individual toner particles each have a reduced surface area and hence a reduced chargeability. Thus, such a finely particulate toner is liable to attachment to non-image area (fog) and toner flying. Such a finely particulate toner exhibits a deteriorated fluidity that makes itself less handleable during transportation or other occasions.
Moreover, such a finely particulate toner exhibits high adhesion and a low impact resistance that can cause carrier stain (carrier spent) and hence reduction of the life of developer. Further, it is rather difficult to remove such a finely particulate toner from the photoreceptor, possibly causing the formation of thin film on the photoreceptor during printing (filming).
Further, such a finely particulate toner requires a greater energy to obtain the same fixing strength than a toner having a greater particulate diameter. Thus, the toner constituting the image can be partially transferred to the surface of the heat roller during fixing, possibly causing offset, i.e., phenomenon involving re-transfer of the toner thus transferred to subsequent recording paper and stain on the image formed thereon. Further, the production of such a finely particulate toner is liable to yield drop at grinding and classification steps that adds to production cost thereof. Thus, such a finely particulate toner is liable to these many troubles. In general, therefore, a toner having a particle diameter of less than 6 .mu.m can be hardly put into practical use. In practice, toner particles are classified into a range of from 6 .mu.m to 10 .mu.m before use.
Nevertheless, even a particulate toner having a particle diameter falling within the above-defined range is liable to the foregoing various troubles. Attempts have been made to overcome these troubles and hence obtain a high precision image at a high reliability. Referring to chargeability for example, as the particle diameter of toner particles is reduced, individual particles each have a reduced chargeability that causes the foregoing troubles. Attempts have been made to secure desired chargeability by enhancing the dispersibility of pigment or charge controller constituting the toner as described in H. T. Macholdt, 1991 transactions of "Japan Hardcopy", page 13, 1991.
As disclosed in JP-B-8-3660 (The term "JP-B" as used herein means an "examine Japanese patent publication"), an attempt has been made to improve the low temperature fixability and offset resistance of toner by using as a binder an aromatic polyester resin comprising an amorphous polymer block and a crystalline polymer block in a proper proportion in its molecule.
Referring to carrier, the reduction of the particle diameter of toner is accompanied by the use of a carrier having a particle diameter as small as not more than 100 .mu.m as calculated in terms of weight-average particle diameter and hence a raised specific surface area that improves the triboelectricity of the toner. However, such a carrier having a particle diameter of less than 40 .mu.m exhibits a lowered magnetic force and thus can easily be electrostatically attracted to the image carrier. Thus, carrier particles are classified into a range of from 40 .mu.m to 100 .mu.m before use.
When the carrier particles fall within this range, the resulting developer has a small particle diameter itself. In the electrophotographic process, a proposal has been made that such a small particle diameter developer not only exhibit an improved chargeability but also enhance the capacity of recycling the toner recovered from the image carrier as disclosed in JP-A-8-15986. With these improvements, more finely particulate toners and developers have been put into practical use in copying machines, printers, etc.
However, when printing is repeated using a practical electrostatic recording apparatus, the foregoing problems characteristic to finely particulate toner, particularly increase in the chargeability of the toner caused by printing, can be hardly avoided, making it impossible to keep the image density to a proper range.
In order to control the chargeability of toner to a proper range, it is necessary to properly select the kind and amount of the charge controller to be incorporated in the toner. It is further important to attach a fluidizing agent to the surface of toner so that the fluidity of the toner is improved to unify the chargeability of the surface of the toner. As such a fluidizing agent there may be used a particulate inorganic material such as silica powder, alumina powder and titania powder or a particulate organic material such as acrylic resin powder and polyamide resin powder, most normally silica powder. However, the improvement of the fluidity of toner alone cannot stabilize the chargeability of the toner.